Printed electronic features, such as thin film transistor (TFT) electrodes, conductive line, trace, pad, and interconnect for integrated circuits, and radio frequency identification (RFID) technology, are an area of intensive research. The ability to directly print electronic features opens the door to a myriad of low-cost flexible electronics with many possibilities for application.
Materials commonly used for printing electronic conductive features include metal materials. In particular, nanoparticulate metal materials are widely-used in printed electronics applications because they have superior characteristics that yield a better product. Metallic nanoparticles are particles having a diameter in the submicron size range. Nanoparticle metals have unique properties, which differ from those of bulk and atomic species. Metallic nanoparticles are characterized by enhanced reactivity of the surface atoms, high electric conductivity, and unique optical properties. For example, nanoparticles have a significantly lower melting point than bulk metal, and a lower sintering temperature than that of bulk metal. The unique properties of metal nanoparticles result from their distinct electronic structure and from their extremely large surface area and high percentage of surface atoms.
Metallic nanoparticles are either crystalline or amorphous materials. They can be composed of pure metal, such as silver, gold, copper, etc., or a mixture of metals, such as alloys, or a core of one or more metals such as copper covered by a shell of one or more other metals such as gold or silver. Nickel has been used for conductive inks for a very limited extent because of its relatively low conductivity (approximately four times less than that of copper or silver). Gold and silver can provide good conductivity, but are relatively expensive. Moreover, gold and silver require high temperatures for annealing, which can pose a challenge for printing on paper and plastic substrates. Copper provides good conductivity at a low price (about one percent of that of silver). Unfortunately, copper is easily oxidized and the oxide is non-conductive. Conventional copper-based nanoparticle inks are unstable and require an inert/reducing atmosphere during preparation and annealing in order to prevent spontaneous oxidation to non-conductive CuO or Cu2O. Copper polymer thick film inks have been available for many years and can be used for special purposes, for example, where solderability is required. Another interesting strategy is to combine the advantages of both silver and copper. Silver plated copper particles are commercially available, and are used in some commercially available inks. Silver plating provides the advantages of silver for inter-particle contacts, while using the less-expensive conductive metal (copper) for the bulk of the particle material. However, as discussed above, silver is relatively expensive, and using a plating method to obtain core-shell structures is not cost-effective. Thus, there is a need for a reliable, cost-effective method for producing inks that are suitable for printing electronic features.